In proton exchange membrane (PEM) fuel cells, hydrogen is the anode reactant and oxygen is the cathode reactant. The anode and cathode are made of finely divided catalytic particles, which are typically of costly precious metal. The membrane electrode assemblies are relatively expensive to manufacture and require certain conditions for effective operation. For example, carbon monoxide poisons the platinum catalyst typically used in the anode and cathode. In order to reduce catalyst loading, the CO level of the anode fuel should be as low as possible, preferably 5 ppm or less.
For vehicular applications, it is desirable to use a liquid fuel such as hydrocarbons (such as gasoline), alcohols (for example methanol or ethanol), and/or mixtures thereof (for example blends of ethanol or methanol and gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. Gaseous fuels such as methane, natural gas, or propane may also be used. Dissociation of such fuels to produce hydrogen is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam (and sometimes air) to yield a reformate gas containing primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water are ideally reacted to generate hydrogen and carbon dioxide. In practical application, the reactors inevitably produce small quantities of carbon monoxide, which must be removed before the hydrogen containing stream is fed as anode reactant to a PEM cell. For example, in a gasoline reformation process, steam, air and gasoline are reacted in a primary reactor. In a first reaction, air reacts with the fuel exothermally, while in a second reaction steam reacts with the fuel endothermically. The primary reactor produces hydrogen, carbon dioxide, carbon monoxide, and water.
Reactors downstream of the primary reactor are generally required to lower the CO concentration in the hydrogen-rich reformate to levels tolerable in the fuel cell stack. Downstream reactors may include a water gas shift reactor (WGS), a preferential oxidation reactor (PROX) or series of these reactors. The water gas shift reactor produces additional hydrogen from water and carbon monoxide. The outlet carbon monoxide concentration from the WGS reactor is limited by the thermodynamic equilibrium of the water gas shift reaction. The preferential oxidation reactor selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide using oxygen from air as an oxidant. Control of air feed is important to selectively oxidize CO to CO2. The preferential oxidation reactor is not 100% selective, and results in consumption of hydrogen. The heat generated from the preferential oxidation reactor is at a low temperature, resulting in excess low-grade heat that is difficult to integrate into the fuel processor system.
In application Ser. No. 09/780,079 filed Feb. 9, 2001, a pressure swing adsorber (PSA) is described and a method for reducing the CO level of a hydrogen-containing stream is provided. The pressure swing adsorber can purify a reformate stream or the output of a water gas shift reactor to a near pure hydrogen stream with a carbon monoxide level acceptable for use in PEM cell. The pressure swing adsorber does not consume hydrogen, but must be sized appropriately to lower the CO concentration below 5 ppm before input into the fuel cell. It would be desirable to provide a system for reducing CO so that the size of any such adsorber may be minimized.